Tire pressure monitoring systems have been widely used in many vehicles such as cars that monitor current gas pressures (tire pressure) in tires and notify the driver of warnings if the pressure in any tire is decreased. In such a known tire pressure monitoring system, wireless pressure detectors are installed inside the tires and on the outer circumference surfaces of the wheels. The pressure detectors transmit detection results of the tire pressures wirelessly to an external controller (pressure monitoring device), and the controller determines the received tire pressures and, as necessary, notifies the driver of warnings of under inflation (see, for example, Patent Literature 1 and 2).